1. Field of the Invention
The present invention relates to a chopper amplifier which amplifies a finite, low-frequency signal.
2. Description of the Related Art
In related art Japanese Patent Application Publication No. 2006-279377 discloses a chopper amplifier to amplify a finite, low frequency detection signal, for example.
Such a chopper amplifier comprises a chopper modulator to modulate a finite, low-frequency input signal by a certain control signal and output a modulation signal, a differential amplifier to differentially amplify the modulation signal, and a chopper demodulator to demodulate the differentially amplified modulation signal in accordance with the control signal for output.
FIG. 5 shows an example of typical chopper amplifier and FIGS. 6A to 6F show voltage waveforms and frequency spectra at nodes of the chopper amplifier in FIG. 5. FIG. 6A show gate control signals φ1, φ2. A chopper modulator 1 and a chopper demodulator 2 in FIG. 5 each comprise NMOS transistors. The NMOS transistors become conductive when high-level gate control signals φ1, φ2 are applied to their gates while they become non-conductive when low-level signals are applied thereto.
FIG. 6B shows a voltage (signal) S1 from a signal source and a voltage B1 from a bias source. The bias source constantly outputs a DC voltage to supply a bias voltage to the amplifier. FIG. 6C shows a frequency spectrum of the signal S1, and FIG. 6D shows the output of a modulator, that is, a signal obtained by modulating the voltage waveform in FIG. 6B by the chopper modulator 1. FIG. 6E shows that the signal S1 with a frequency component fs is converted into a signal with a frequency component fchop±fs, 3fchop±fs, . . . .
In FIG. 6F the signal is amplified by the amplifier with a gain A. FIG. 6G shows the spectrum of the output voltage of the amplifier, and it can be seen therefrom that a 1/f noise and the signal of the amplifier are frequency-separated. In FIG. 6H the output voltage of the amplifier is demodulated by the chopper demodulator 2. In FIG. 6I the frequency of the signal component is returned to the one before the conversion and the 1/f noise of the amplifier is converted by the chopper demodulator 2 to be in a frequency band near the fchop.
In FIGS. 6J, 6K only the 1/f noise is cut off by a lowpass filter (LPF) and the signal S1 is output from the signal source without change. That is, the chopper amplifier works to move the signal in a low frequency range to a high frequency range, separate the frequency band of the signal component and the 1/f noise, and amplify the signal.
However, there is a problem with the chopper amplifier that a residual offset applied to a voltage of the output OUT of the LPF due to clock feedthrough occurring when the Nch transistor switches of the chopper modulator 1 are transited from a non-conductive state to a conductive state.
This is mainly caused by a mismatch between the output resistances of the signal source and the bias source, that between the gate-source capacitances of the transistor switches, or that between the input capacitances of the two input terminals of the amplifier. An example is described below.
FIG. 7 is a circuit diagram of a chopper amplifier by way of example, showing an output resistance R1 of a signal source S1, an output resistance R2 of a bias source B1, gate-source capacitances Cgs of four Nch transistors of a modulator, and input capacitances C of + and − input terminals of an amplifier. For the sake of simplicity, assumed that the gate-source capacitances Cgs of the transistor switches are the same and so are the input capacitances C of the + input terminals, and a mismatch occurs only between the output resistances R1 and R2, and R1<R2. When the switches inside the modulator are shifted from a non-conductive state to a conductive state, the gate potentials of the Nch transistors of the switches transit from GND to VDD, injecting a charge into a source (clock feedthrough). Because of this, the output voltages of the signal source S1 and bias source B1 rise by Ving for a moment where Ving=Vdd(Cgs/(Cgs+C)). The charge injected into the capacitance C by the clock feedthrough is discharged towards the signal source and bias source. In FIG. 8B transient responses of the signal source and the bias source when the charge is discharged is shown. Since the output resistance R2 of the bias source B1 is larger than that R1 of the signal source S1, it takes a longer time for the output resistance R2 to settle than the output resistance R1. This causes a spike in the output waveform of the amplifier immediately after the switching of the modulator switch (FIG. 8D). The spike waveform passes through the demodulator without a change and is applied to the LPF, as shown in FIG. 8E. The voltage that the LPF outputs is a mean value of the waveform in a certain time so that the voltage corresponding to the spike waveform becomes an offset added to the output voltage of the LPF. In FIG. 8F the spike waveform applied to the input of the amplifier appears as a certain residual offset in the output voltage of the LPF.